One type of mass spectrometer that is extensively used for qualitative and quantitative analysis of chemicals employs one or more quadrupole mass filters. In such spectrometer systems, quadrupole mass filters incorporating four conductive metal rods are supported on amounts made of an insulating material. The rods are energized by combined direct current (d.c.) and alternating current (a.c.) voltage to achieve selective mass focusing. An example of a mass filter is described in U.S. Pat. No. 4,032,782 entitled "Temperature Stable Multiple Mass Filter and Method Therefo" which issued to the same assignee. The patent discloses a method of maintaining filter stability by thermal matching of the rods and the mounts.
To obtain accurate readings and interpretations of analyses performed by mass spectroscopy, it is highly desirable that the mass peak waveforms obtained by the scans are smooth and not characterized by spurious splits or depressions which affect the spectral quality of the data. In prior art systems, it has been observed that such spurious splits and depressions of the mass peaks occur frequently, thus deleteriously affecting the interpretation of the resulting data.
We have discovered that by unbalancing the r.f. voltages applied to the pairs of quadrupole rods that spurious splits and depressions are substantially reduced.